I used to love low-level graphics programming (drawing polygons, fractals), closely worked with people doing kernel stuff, have a CS degree in Machine learning, doing infosec for fun and have dealt with quite a bit of distributed/system/database stuff to scale.

After some time, these challenging stuff have the same problem, being mundane. Specially constraint of work environment and people you have to deal with. Plus some of the "coolest stuff" just blows up with overfitting and seems to be a mere hype to me.